Sulfur is an objectionable element that is typically found in fossil fuels, where it occurs both as inorganic sulfur, such as pyritic sulfur, and as organic sulfur, such as a sulfur atom or moiety present in a wide variety of hydrocarbon molecules, including for example, mercaptans, disulfides, sulfones, thiols, thioethers, thiophenes, and other more complex forms. Crude oils can typically contain, for example, amounts of sulfur up to 5 wt % or more.
The presence of sulfur in fossil fuels has been correlated with the corrosion of pipeline, pumping, and refining equipment, and with the premature breakdown of combustion engines. Sulfur also contaminates or poisons many catalysts which are used in the refining and combustion of fossil fuels. Moreover, the atmospheric emission of sulfur combustion products, such as sulfur dioxide, leads to the form of acid deposition known as acid rain. Acid rain has lasting deleterious effects on aquatic and forest ecosystems, as well as on agricultural areas located downwind of combustion facilities. To counter these problems, several methods for desulfurizing fossil fuels, either prior to or immediately after combustion, have been developed.
One recently developed technique for desulfurizing fossil fuels is known as biodesulfurization (BDS). BDS is generally described as the harnessing of metabolic processes of suitable bacteria to the desulfurization of fossil fuels. Thus, BDS typically involves mild conditions, such as ambient or physiological temperature and pressure, and does not involve the extremes of temperature and pressure associated with conventional desulfurization technologies. Kilbane, U.S. Pat. No. 5,104,801 describes one such process wherein a mutant Rhodococcus rhodochrous strain ATCC No. 53968 selectively cleaves the C--S bond in organic carbonaceous materials.